The terms base station, wireless transmit/receive unit (WTRU) and mobile unit are used in their general sense. As used herein, a wireless transmit/receive unit (WTRU) includes, but is not limited to, a user equipment, mobile terminal, mobile station fixed or mobile subscriber unit, pager, or any other type of device capable of operating in a wireless environment. WTRUs include personal communication devices, such as phones, video phones, and Internet ready phones that have network connections. In addition, WTRUs include portable personal computing devices, such as PDAs and notebook computers with wireless modems that have similar network capabilities. WTRUs that are portable or can otherwise change location are referred to as mobile units. When referred to hereafter, a base station is a WTRU that includes, but is not limited to, a base station, Node B, site controller, access point, or other interfacing device in a wireless environment.
In a third generation partnership program (3GPP) or 3GPP-like system, time division duplex (TDD) wireless communications are encoded with scrambling codes, spreading codes and predetermined training sequences known as midambles, which are helpful to reconstruct the originally transmitted signals. Each base station cell uses a unique scrambling code to distinguish base stations in the network when establishing a link between a particular base station and the WTRUs that it serves. Spreading codes are associated with spreading each WTRU's data into pieces across the same frequency bandwidth as the other WTRUs, while tagging each respective data signal with a unique spreading code to permit reconstructing the data at the receiver. The midamble is a designated portion of a time division channel timeslot containing a known code sequence that is used at the receiver during channel estimation.
In many wireless communication systems, many communications may share the same radio frequency spectrum. When receiving a specific communication, all the other communications using the same spectrum cause interference to the specific communication. As a result, increasing the transmission power level of one communication degrades the signal quality of all other communications within that spectrum. However, reducing the transmission power level too far results in undesirable received signal quality, such as measured by signal to interference ratios (SIRs) at the receivers. In such systems, transmission power control algorithms are used.
Various methods of open and closed loop power control for wireless communication systems are known in the art. The purpose of such systems is to rapidly vary transmitter power in the presence of a fading propagation channel and time-varying interference to minimize transmitter power while insuring that data is received at the remote end with acceptable quality. One approach is to divide transmission power control into separate processes, referred to as outer loop power control (OLPC) and inner loop power control (ILPC).
In outer loop power control, the power level of a specific transmitter is based on a target SIR value. As a receiver receives the transmissions, the quality of the received signal is measured. The transmitted information is sent in units of transport blocks (TBs), and the received signal quality can be monitored on a block error rate (BLER) basis. The BLER is estimated by the receiver, typically by a cyclic redundancy check (CRC) of the data. This estimated BLER is compared to a target quality requirement, such a target BLER, representative of quality of service (QoS) requirements for the various types of data services on the channel. Based on the measured received signal quality, a target SIR adjustment control signal is sent to the transmitter. The transmitter adjusts the target SIR in response to these adjustment requests.
In 3GPP wideband code division multiple access (W-CDMA) systems utilizing time division duplex (TDD) mode, the network sets the initial target SIR to the WTRU at the call/session establishment and then subsequently continuously adjusts the target SIR of the WTRU during the life term of the call as dictated by the observation of the uplink (UL) BLER measurement.
In inner loop power control, the receiver compares a measurement of the received signal quality, such as SIR, to a threshold value (i.e., the target SIR). If the SIR exceeds the threshold, a transmit power command (TPC) to decrease the power level is sent. If the SIR is below the threshold, a TPC to increase the power level is sent. Typically, the TPC is multiplexed with data in a dedicated channel to the transmitter. In response to received TPC, the transmitter changes its transmission power level.
FIG. 1 shows a block diagram of a portion of a wireless network configuration, including base stations BS1 and BS2, and WTRU1, WTRU2 and WTRU3. The base stations are the link between the communication network and the WTRU. The network (not shown) is responsible for routing information to its correct destination, overseeing multiple base stations, managing radio resources within the geographic area of wireless radio service coverage serviced by the base stations and controlling the physical radio resources for the interface between the base station and WTRU. Base station BS1 transmits in region 11, base station BS2 transmits in region 12, and region 13 represents an overlapping region between the two base stations, in which WTRU1 resides. In this example, WTRU1 is mobile and moving from region 11 to region 12. As such, WTRU1 is a candidate for handover, while WTRU3 in region 11 and WTRU2 in region 12 are positioned to communicate adequately with their respective affiliated base stations BS1 and BS2.
FIG. 2 shows a block diagram of a portion of a wireless network operating under softer handover. In softer handover, two or more sectors of a single base station transmit and receive a WTRU's signal. Here, base station BS transmits and receives according to sector 21 and 22. While WTRU2 resides in sector 22, and WTRU3 resides in sector 21, they have no need for handover as their positions allow for efficient communication with each of their respective base station sectors. However, WTRU1 resides in overlapping region 23, which falls within both sector 21 and sector 22. Therefore, WTRU1 is a candidate for softer handover as it travels between sectors 21 and 22.
The need to manage and control handover of WTRUs is of paramount importance in telecommunication networks. It is known that soft handover (SHO) is used in FDD-CDMA networks, including IS-95, CDMA 2000 and 3GPP WCDMA. Soft handover can improve performance when the WTRU is located where comparable power is received from two or more base stations (BSs).
Currently, only hard handover is supported in the standardized 3GPP TDD WCDMA systems, for both high and low chip-rate variants. In hard handover, transmission and reception of signals between WTRU1 and base stations BS1 and BS2 do not occur as smoothly as in soft handover, or between sectors 21 and 22 as in softer handover. In hard handover, the communication transition to the second base station or sector can only commence if the first communication is terminated with WTRU1. Providing a method to implement soft handover in TDD CDMA networks would yield increased capacity and coverage.